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In Reply to: RE: Link for the above posted by Bibo01 on May 10, 2012 at 00:59:19:
The interesting difference is the difference between A and B. This is what the listener might hear. If the DAC had complete linearity, this difference would be at -138 dB, and would not easily be detected by the measurement setup that was used, which as not so good as -100 dB. The measurement setup is not adequate to measuring the actual differences in the two signals, as can be seen by seeing how little difference there are between the curves. (Also, without knowing the details of the FFT, e.g. the window size/shape, it is not possible to correctly interpret levels in frequency spectrums. This can easily be seen by running a spectrum plot on a given waveform using different FFT sizes and windows.)
If one wants to see the magnitude of difference in the analog output of the DAC with two waveforms it will be difficult to measure differences that are well below the noise floor of the system, e.g. 38 dB below -100 dB. While this is difficult, it is not impossible, as it is possible to take multiple measurements and average over them, but this will require careful design and calibration of test equipment, as the averaging has to be synchronous. Null test measurements tend to be unstable, because they are a difference of large quantities. (This is easily seen when doing null-test differences of sample rate converters where sub-sample offset errors often prevent deep nulls.)
In general, the test equipment needs to be ten times better than the equipment under test, otherwise evaluation of experimental results are difficult. This is a problem when evaluating "state of the art" cost no object equipment because the device under test is going to be as good as the measurement equipment. For measuring high quality equipment specialized test procedures are required. Note only is it difficult to do these measurements, we don't understand what the differences are that ought to be measurement. We are looking for a needle in a haystack and all we have is a dim light.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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Follow Ups
- Null Test Differences (NTD) are problematic - Tony Lauck 08:40:43 05/10/12 (3)
- RE: Null Test Differences (NTD) are problematic - Bibo01 15:15:42 05/10/12 (2)
- RE: Null Test Differences (NTD) are problematic - Tony Lauck 15:28:30 05/10/12 (1)
- RE: Null Test Differences (NTD) are problematic - Bibo01 15:39:58 05/10/12 (0)
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